1. Field of the Invention
The present invention relates to a wind turbine generator system and particularly to a wind turbine generator system capable of protecting a converter connected to a rotor of an AC-excited generator from a voltage drop due to a power failure in a grid.
2. Description of the Related Art
An AC-excited generator (Doubly-Fed Induction Machine) in a power generating system can equalize in frequency a generated voltage at a stator to a grid by AC-exciting a rotor at a slip frequency (difference between the grid frequency and a rotational frequency). The variable exciting frequency of the rotor (slip frequency) allows a revolution speed of the wind turbine to be variable as well as an exciting converter for the rotor to be smaller in capacity than other type of electric generators.
However, when a voltage drop in a grid occurs due to an earth fault, the AC-excited generator tends to supply a power to the place where the earth fault occurs. In the event, an excessive current is induced in rotor windings, which causes an excessive current in an exciting converter connected to the rotor. Accordingly, a device, called Crow-bar, for short-circuiting a rotor circuit with thyristors may be used.
In Europe and other countries there are regulations which require that wind turbine systems continue to operate without disconnection from the grid. Thus, when a short-time voltage drop occurs, the wind turbine generator systems are required to restart power generation after power failure without disconnection from the grid to minimize influence on the grid.
Conventionally, the AC-excited generators are mainly used in large-scale power generation systems such as pumped storage power plants. An operating method of the AC-excited generators is known in which, upon a large scale power failure in the grid, a rotor circuit is short-circuited with externally excited devices.